1. Field of the Invention
The present invention generally relates to methods and apparatuses for detecting the potential for erroneous detection of a frequency in a signal. More particularly, it relates to an efficient harmonic analysis technique for detecting the presence of particular frequencies in a composite signal to recognize the potential for erroneous detection of an alerting or other tone.
2. Background of Related Art
While the present invention relates to the suppression of erroneous tone detection in general, it and its background are described with respect to a particular embodiment useful for detecting alerting tones relating to the reception of call related information, e.g., Caller ID information.
As is well known, Caller ID services permit a telephone customer to learn the identity of, or at least the originating telephone number of, a party seeking to place a telephone call to the customer. An older service referred to as Call-Waiting (CW) alerts a customer engaged in a telephone connection to another party, that a third party desires to place a telephone call to the customer. A newer service offered in telephone systems combines caller identification and call-waiting services, to not only alert a customer during a telephone connection that a third party desires to establish a connection, but also transmits to the CPE, caller identification information identifying the telephone number and/or name of the third party. This latter service is sometimes referred to as CIDCW (caller ID and call waiting) or Caller ID Type II.
Participation in a system with Caller ID and Call Waiting requires not only the presence of a central office capable of providing such service, but also CPE capable of receiving and transmitting the necessary control, acknowledgment and data signals. The general sequence of events during a telephone call in which the customer has Caller ID and Call Waiting service is as follows.
When a customer is currently engaged in a telephone conversation with a second party, a third party desiring to complete a telephone call with the customer may dial the customers number. During the customers conversation with the second party, the central office sends a subscriber alerting signal (SAS) alerting the customer that a third party desires to make a connection with the customer. The subscriber alerting signal is typically a single tone 400-hertz signal of about 500 milliseconds in duration, that is audible to the customer.
The central office follows the transmission of the subscriber alerting signal with the transmission of a CPE alerting signal (CAS). Similar to a dual tone multi-frequency signal, the CPE alerting signal has 2130 hertz and 2750 hertz components and optimally lasts for 80 milliseconds. Upon detection of the CPE alerting signal, the CPE sends a CAS acknowledgment signal (CAS ACK) to the central office, signaling to the central office that the CPE is ready to receive Caller ID information regarding the third party.
At the time the CPE transmits the CAS acknowledgment signal, it also disables the voice transmission transducer in the CPE. While the central office maintains the connection with the second party, it suppresses the CAS acknowledgment signal from the transmission sent to the second party. When the central office receives the CAS acknowledgment signal, it sends the caller identification information to the customer in the form of a frequency shift keyed (FSK) signal. The CPE can then display the caller identification information on a display unit as is known in the art. Full communication is established with the second party after receipt of the caller identification information. The customer, through the CPE can then decide how to handle the third party, such as placing the second party on hold and establishing the connection with the third party, for example.
Because of the proximity of the CPE alerting signal to voice frequencies transmitted and received in typical telephone systems, reliable detection of the CPE alerting signal can sometimes be a problem in conventional CPEs. For instance, conventional tone detectors used in this capacity sometimes experience xe2x80x9ctalkoffxe2x80x9d episodes in which CPE alerting signals are falsely detected, and xe2x80x9ctalkdownxe2x80x9d episodes in which actual CPE alerting signals are missed due to interference with speech. This erroneous operation is often the result of harmonic components of speech signals occurring in the frequency ranges of a tone signal such as the standard CPE alerting signal.
One conventional approach to eliminating talkoff and talkdown episodes during CPE alerting signal detection is described in U.S. Pat. No. 5,263,084 issued Nov. 16, 1993 to Chaput et al. The Chaput approach appears to be as follows. When the CPE receives the subscriber alerting signal, it mutes its handset for a predetermined period of time to reduce or eliminate background noise, in expectation of the occurrence of a CPE alerting signal conforming to known standards. The muting interval must be chosen to be of sufficient duration to reliably allow silence for CPE alerting signals of a given tolerance (i.e., beginning time, duration). Unfortunately, this technique requires a priori knowledge of the occurrence of an incoming tone signal. Moreover, the muting interval is typically long enough to be noticed by and cause an annoyance to customers engaged in a telephone conversation.
A different conventional approach employs the use of brute-force digital signal processing (DSP) methods, such as the use of fast Fourier transforms (FFTs), to continuously analyze each frame of a digitized signal for the presence of any/all expected tones.
The operation of a conventional frequency detector and suppresser is shown in FIGS. 5A to 5D.
In particular, a series of four contiguous data frames are shown in FIGS. 5A, 5B, 5C and 5D, respectively. A Fast Fourier Transform is performed on each data fame, and each data frame is then tested for each particular frequency which may cause talkoff. Thus, each of the four data frames shown in FIGS. 5A to 5D, respectively, are analyzed by, e.g., a complete set of tone detectors 402-412 relating to tones which might be expected on the signal line.
Using this conventional approach, there is no need for muting to eliminate background noise. However, such an approach is extremely resource-intensive, requiring a great number of iterative calculations, and large memory requirements, which may increase the cost of practical Caller ID- and Call Waiting-capable CPE beyond acceptable levels.
Thus, there is a need for a reliable tone detector (i.e., one which eliminates talkoffs and talkdowns) for detecting CPE alerting signals or other tones without the need for annoying muting intervals, and without using cost prohibitive hardware where intensive digital signal processing methods are utilized to continuously search for any/all present tones.
In accordance with the principles of the present invention, a false tone detect suppresser comprises a frequency bin grouper adapted to group transformation information into a plurality of frequency bins each relating to a particular frequency in a composite signal. A frequency bin analyzer is adapted to analyze information in only a sub-plurality of the plurality of frequency bins during any particular data frame to provide a basis for indicating a presence of a harmonic relating to a tone being detected in the composite signal.
A method of suppressing a false tone in a composite signal including a speech related component in accordance with another aspect of the present invention comprises transforming the composite signal into a plurality of data frames each containing information relating to a plurality of frequency bins. The plurality of frequency bins are grouped such that fewer than all of the plurality of frequency bins are analyzed in any one of the plurality of data frames. One of the groups are analyzed during each of the plurality of data frames for a presence of associated frequency components such that a sweeping harmonic analysis of the speech related component in the composite signal is performed once over the plurality of data frames.
A tone detection method for detecting a specified tone signal in an input signal containing background noise in accordance with yet another aspect of the present invention comprises transforming the input signal into successive frames having a predetermined number of frequency bins. The successive frames are partitioned into a predetermined number of frequency bins. The frequency bins of a frame are grouped into a predetermined number N of multiple groups, each group corresponding to one of several contiguous temporal windows defined for all frames. A group of frequency bins are analyzed according to predetermined sound algorithms during a predetermined analysis period during a temporal window. A presence of the specified tone signal is indicated. For each successive frame, a group of frequency bins corresponding to a temporal window contiguous to the temporal window considered for the previous frame is analyzed until a frequency bin analysis is performed for every predefined temporal window.